I saw District 9 and I freaked out like everyone else did and Neill [Blomkamp] was instantly top of the list of people I wanted to work with. Then I heard about this movie.

I didn’t think I would get to work with him right away but sure enough the next movie there was this role available. And about 20 minutes into my first meeting with Neill — I met him for coffee, in a diner in New York — he pulls out these renderings that he has done. He has built the entire world on his computer and he has made a graphic model and there was a whole other book he’d got of weaponry. This is stuff that isn’t necessarily going to be featured in the movie. It is deep background stuff but each gun’s got a description of how it is engineered and how it works. The level of detail was literally mindboggling.

Apart from speaking to James Cameron about Avatar, that was really the only other time I have met a director who did that, who literally came with a fully formed concept, a science fiction world, which was unlike any world I had ever seen.

You’ve said you’d like to direct films yourself one day, so what did you pick up from Neill on a practical level?

Practically speaking it is the same thing that every director talks about, which is just the level of preparation, knowing in intimate detail the movie that you are making. There are other directors I have worked with who aren’t making a science fiction film, creating a whole new world, but who are still aware of all the detail and are capable of explaining the story that they are telling perfectly.

That’s why directors’ interviews are so much better than actors’ because they are completely aware of why they’ve made the movie and what the movie is to them. So you get specific answers to your questions.

You shot some extensive scenes in an enormous garbage dump in Mexico City…

Yes, a number of locations were within the dump. We had locations; for instance, one of them was called ‘Poo River.’ Literally, it was ‘Okay, can we get everybody down to Poo River for scene 36?’ And only after we’d started filming do they tell you that all the dust in the air is really faecal matter!

Presumably Diego Luna took you out to some great places in Mexico City to compensate?

Absolutely. He took us to the nicest restaurants. We got to see the good side of Mexico City, definitely. We didn’t stay at the dump overnight!

We would leave, we’d go home, we’d shower, and then we’d go to the best restaurants in the world, thanks to Diego. We had a great time in Mexico City.

For all the bad smells, the garbage dump must’ve given Elysium some really authenticity?

Neill spoke very articulately about why we needed to do that and it is a very convincing argument and he was right.

It is the visual expression of the entire theme. We have a Bugatti space craft which crashes in this Third World dump, so it is a really arresting visual collision of these two worlds and also it is a completely unique place to do an action set piece. It was an incredible back drop to use. We knew why we were there and, yes, it was a little uncomfortable for a couple of weeks but the good far out- weighed the discomfort.

Were those the toughest scenes to shoot?

No, that wasn’t the hardest location. There was a set in Vancouver. At the end of the third act, Sharlto [Copley, who plays the villainous Kruger] and I face off on this gantry in the inner core of Elysium. There you get under the surface and see the mechanics of how it works and that set was great.

It was beautifully built and really interesting, but the scene required us to be wet because we had walked through these hoses. So Sharlto and I were both wet and they had these big ‘Ritter’ fans blowing. It was like a giant wind tunnel almost. So these ‘Ritter’ fans were blowing on to us and we were soaking wet and it was freezing. That was definitely the most uncomfortable stuff. Wagner [Moura, who plays Spider in the movie] was in the scene and he was sick. He ended up with pneumonia. And he is a tough guy.

I saw him and he was green and I said, ‘Are you all right, man?’ And he looked down and he looked up and he said, ‘I am not well.’ They got him a little tent and put heaters in it so in between takes he could run in there and get dry and warm up. It is what it is at that point.

How did you enjoy your character’s look in the film with the skinhead and tattoos?

When I had tattoos all over my neck it definitely made me feel different when I was walking around. But, again, it was Neill who gave me pictures of Max and he was this guy who is bald and he had all these big muscles and tattoos everywhere and so it was really easy to go, ‘Okay, well!’ They hired a trainer for me and I went to the guy with the picture and I went, ‘Just do that!’ And he goes, ‘Okay, I got it. That’s a lot of work.’

So it was really just the pain of dieting and just spending a lot of time in the gym. The gym part is kind of cool — when else do you get to indulge like that? Unless you are an athlete or a personal trainer, no one spends four hours a day in the gym. So that part of it was really indulgent and fun. But not eating what you want and not drinking red wine whenever you wanted, that was a total pain.

When shooting you were flying home to see your family at weekends when you could. Do you do that on most movies?

It really depends on the movie. With Elysium we shot it over the summer during the school holidays, so the whole summer we were in Vancouver together.

Then, when they started school, I still had some time to do in Vancouver and that was when I was taking the red-eye flights back home. Then we went to Mexico and they were completely in school at that point. We have a ‘two-week rule’ so after two weeks I went back home for three days and then I came back and finished in Mexico.

When I did True Grit, which was a much smaller commitment time-wise for me, I think I shot a total of 25 days over the course of a 70-day movie. I literally said to the Coens brothers, ‘My only stipulation for doing the movie is that at least every two weeks I can get home,’ and they took it a whole step further where I was working three days on and then I’d have three days off. I was commuting to Texas and the kids didn’t feel my absence.